The present invention relates generally to anti-reflection coatings for optical elements that transmit electromagnetic radiation in two different wavelength regions, e.g., in both the millimeter (mm) region and the long wave infrared (LWIR) region.
When designing optical systems for simultaneous use in the mm-wave (xcx9c3 mm) and LWIR (xcx9c10 xcexcm) spectral regions that employ refractive materials, it is necessary to suppress Fresnel reflection losses at the interfaces of high refractive index regions. Since optimum materials (particularly for immersion lens applications) have refractive indices that are often greater than 3 or 4, these Fresnel losses can be very high (approaching 40% per surface) if anti-reflection coatings are not used. The present invention provides for a dual-color, mm-wave and LWIR, anti-reflection coating that is simple, practical, low cost and high performance.
Antireflection coatings for both mm-wave and LWIR separately are well known. Individually, these may take the form of dielectric layers or surface micro-structures (such as crossed grating structures.) Only recently has the idea of combining both simultaneously in a single sensor been raised, largely based on the attempts to develop two-level, micro-structure and macro-structure, micro-bolometer detector arrays. However, the present inventor is unaware of any prior art in the area of anti-reflection coatings for simultaneous mm-wave and LWIR imaging.
In accordance with the present invention, a multi-layer anti-reflection coating for simultaneously coupling electromagnetic radiation of two different wave-lengths, xcex1 and xcex2, where xcex1 is greater than xcex2, from a first region into a second region is provided. The multi-layer anti-reflection coating comprises:
(a) a first layer having a first thickness and a first index of refraction, a second layer having a second thickness and a second index of refraction, and a third layer having a third thickness and a third index of refraction, wherein the first layer is exposed to a first region having a fourth index of refraction and wherein the third layer is deposited on an optical element comprising the second region and having a fifth index of refraction;
(b) the first layer coupling the radiation from the first region into the second layer, the first layer having an optical thickness of xcex2/4;
(c) the second layer being positioned between the first and third layers, the second layer having a thickness greater than either of the first and the third layers, the second layer forming the anti-reflection coating for the radiation of xcex1 and coupling the radiation from the first region into a second region comprising the optical element, wherein the fourth index of refraction is smaller than the fifth index of refraction; and
(d) the third layer coupling the radiation from the second layer into the second region, the third layer having an optical thickness of xcex2/4, the first layer and the third layer forming the anti-reflection coating for the radiation of xcex2.
Also in accordance with the invention, a method of reducing Fresnel surface losses for two widely separated wavelengths is provided. The method comprises forming the multi-layer anti-reflection coating on the optical element to provide the coated optical element by the process of:
(a) providing the optical element;
(b) forming the third layer on top of the optical element;
(c) forming the second layer on top of the third layer; and
(d) forming the first layer on top of the second layer.
The common use of immersion lenses makes the need for anti-reflection coatings very critical. The present invention is simple, low cost, and provides high throughput.